The invention relates to a positioning device for the magnetic head of a drive mechanism for a magnetic disc store, which device comprises a stepping motor which is supported in to the chassis of the drive mechanism and whose rotor shaft moves a magnetic-head support in a radial direction relative to the magnetic-disc store.
Such a positioning device is known from U.S. Pat. No. 3,678,481. For positioning the magnetic head the rotor shaft of the stepping motor has been extended and is constructed as a spindle. This spindle cooperates with the magnetic-head support in such a way that, depending on the direction of rotation of the rotor shaft, the support is moved radially across the magnetic-disc store. The stepping motor itself is secured to the chassis of the drive mechanism at a different location than the magnetic-disc store. The chassis is generally made of aluminium or a glass-fibre reinforced plastic.
For exactly recording or reading the information on the magnetic-disc store, it is essential that there be no difference in temperature, which results in different expansions between the drive of the magnetic-head support and the magnetic-disc store. The known construction does not allow complete temperature compensation because the magnetic-disc store and the chassis are made of different materials with different thermal-expansion coefficients. For example, in drive mechanisms for flexible magnetic-disc stores, temperatures of 70.degree. C. to 90.degree. C. occur at the location of the housing of the stepping motor. However, for magnetic discs, a maximum temperature of only 50.degree. C. is permissible, which then virtually corresponds to the maximum permissible chassis temperature.
The stepping motors are arranged behind the magnetic disc store for reasons of magnetism and equipment standards. As a result of this, the magnetic head in known drive mechanisms without temperature compensation may exhibit an outward deviation relative to the track on the magnetic-disc store, which deviations may exceed 30 .mu.m at the specified operating temperature. A reliable operation over the entire temperature range then becomes problematic.